blood urea nitrogen analysis serum urea nitrogen analysis 2012-08-08T02:17:44Z A test which measures the amount of nitrogen in the form of urea present in serum, the clear liquid that separates from blood after it has clotted completely. Urea is the chief nitrogenous end product of protein metabolism and is formed in the liver from amino acids and from ammonia compounds and is found in urine, blood, and lymph. JSmith MMO:0000274 measurement_method_ontology